In general, an LCD apparatus has advantages over a CRT (cathode ray tube) type display apparatus in respect of its low weight and small size. The advantage of an LCD derives from its use of liquid crystal for providing images. The liquid crystal is controlled by an electric field. Under an applied electric field, liquid crystal molecules are oriented in a predetermined direction parallel to a direction of the electric field. Light transmittance for providing images varies according to the orientations of the liquid crystal molecules.
The LCD apparatus requires a light source to transmit light through the liquid crystal. The quality of the displayed images depends on a uniformity of the light provided to the liquid crystal, and on a brightness of the light provided to the liquid crystal.
In general, the light source provides a uniform surface light. The light source is typically a backlight assembly. An LGP is a vital component of the backlight assembly. A basic function of the LGP is to change a propagation direction of light introduced into the LGP through a light incident surface thereof, so that the light is emitted from a light emitting surface of the LGP. In one kind of LGP, the light incident surface is roughly perpendicular to the light emitting surface. A simple transparent LGP of this kind is capable of re-directing light, but only to a limited extent. The emitted light provided may have unsatisfactory brightness. Hence, various LGPs having a plurality of tiny reflectors on a surface opposite to the light emitting surface have been devised.
FIG. 11 shows a backlight assembly with another kind of conventional LGP. The LGP includes a cavity formed at a light incident surface thereof. A light emitting diode (LED) is received in the cavity. Thus most light emitted from the LED is introduced into the LGP, and light energy is efficiently utilized. However, a thickness and a weight of the LGP have to be undesirably large. In addition, because the LED is located inside the LGP, it is difficult to dissipate heat generated by the LED. The LED is liable to overheat and fail.
FIG. 12 shows part of another backlight assembly with another kind of conventional LGP. A light coupling element such as a lens is provided between a point light source and the LGP. However, the backlight assembly is not compact, and the relative positions of the various light-emitting and optical components must be fixed with high precision.
What is needed is an LGP having high light energy utilization and a compact configuration.